Anxious - what do I do in this situation??

[WeAreFlute]

I have a dilemma...

I applied to four schools for doctorate in music (flute) programs.  Here are the dates I auditioned:

School 1: February 1

School 2: February 8

School 3: February 13

School 4: February 15

 

So the good news, school 3 gave me an answer almost immediately - I was accepted on February 20, and offered an assistantship on March 4.  School 1 was the next to offer an acceptance on February 26 - no financial offer yet.  School 4 accepted me on March 3, also no financial offer yet, but really hoping this is coming.  I haven't yet heard from school 4, which was the most competitive program of the four I auditioned at.

Here's the problem:

School 3 (first to accept me) indicated that they want my response by March 23!   I don't have an answer from school 4 yet, and I don't feel I can make a good decision without knowing if I've been accepted or not.  I thought that legally, they cannot require me to accept prior to April 15?  Should I just call the school and explain the situation?  I don't want to accept and then back out (if that were the case), likewise, I don't want to put my (so far) only assistantship offer at risk.  

Help!